HTML::Form - Getting ->parse back into $html_document

Hi,

I'm looking to modify a form on the fly before its
presented to the user. Using HTML::Form and push_input I can
do what I want :

@forms = HTML::Form->parse($pagecontent,$pagebase);
$forms[0]->push_input("hidden",{name => 'tucadded',value=>'tboh'});

But how do I get it BACK into $pagecontent?

Thanks, Tuc